The present invention relates to crystal filters, and more particularly, to a monolithic crystal filter to keep a stopband attenuation level at a high level even in a surface installation, and to raise shock resistance with a high electric conductivity. In the present disclosure the test frequency within the stopband is defined as 910 kHz below the center passband f.sub.0, i.e. f.sub.0 -910 kHz. The frequency of f.sub.0 -910 kHz is equal to the secondary lower image of the common intermediate frequency of 455 kHz in a conventional superheterodyne communications equipment.
A monolithic crystal filter (MCF) is utilized to attain desired bandpass filter characteristics, for example, in communication equipment. A MCF is fabricated in general of a piezoelectric element such as a quartz crystal plate configured in a small package suitable for surface mounting and, the highest attainable stopband attenuation level is required for rejection of undesired signals. The stopband attenuation level, however, is governed mainly by the frequency, the dimensions and the fabricated structure of a MCF. The higher the center frequency, the smaller the size of crystal filter, the more complicated the MCF must be by design. The small size, tends to degrade the stopband attenuation level. The stopband attenuation level is reduced at harmonics of resonant frequency of the MCF. Under this condition, although the output signal level is much lower than the input signal level, the signal experiences less than the desired loss between input and output.
Conversely, the inband attenuation level, or the insertion loss, is determined only by the mechanical vibrational efficiency from input to output through a piezoelectric crystal plate at its resonant passband frequencies. Electrical cascade connection of MCFs have been the heretofore best available means for realizing higher stopband attenuation level.
In a MCF, the stopband attenuation level is degraded due to direct electrical transfer past the gap from a first input electrode to a first output electrode spaced apart on a piezoelectric crystal plate. In addition to direct electric transfer, degradation of the stopband attenuation level is also caused by coupling at spurious resonant frequencies of the crystal plate. Japanese Utility Application Sho 60-118993, Japanese Patent Application Sho 63-273981, Japanese Patent Application Hei 1-83007, and Japanese Patent Application Hei 4-46008 disclose devices for improving the stopband attenuation level, particularly in monolithic crystal resonators suitable for surface mounting.
The above prior art, however, has some drawbacks. The stopband attenuation level varies according to the distance between a first input and a first output electrode of a crystal plate and a shield electrode disposed on an insulating substrate. Japanese Patent Application Hei 4-46008 implies an optimum distance resulting in a desired stopband attenuation level, but the relationship between the distance and the electrical grounding of the shield electrode and a metal lid is not disclosed.
Moreover, in the prior art, the crystal plate, fixed by an adhesive at both sides of the terminals extended form the electrodes, generates substantial strain and stress, thereby causing a change in a frequency/temperature characteristic. The frequency/temperature characteristic is the drift of the center of the passband frequency of a filter due to temperature. In fact, even when the adhesive is applied only on one end of the crystal plate to prevent strain and stress in the longitudinal direction, strain and stress are still generated in the transverse direction. When the adhesive is applied at an outlying point, stress and strain are reduced substantially, but the device has decreased supporting strength. Also, in this case, freedom of the other end of the crystal plate generates bar resonant vibration which may cause breakage of the crystal plate or disconnection of a wiring.